1. Field of the Invention
The present invention relates to fractional distillation of chemical compositions and particularly to spinning band fractionating columns in which the band is caused to spin by an external rotating magnetic field.
2. Relation to the Prior Art
Effective separation by distillation techniques of close boiling (5.degree.-10.degree. C.) liquid mixtures which have volumes in the 0.5 to 5 ml. range currently involves the use of sophisticated and expensive equipment. Alternative routes utilizing preparative gas chromatographic instrumentation present an equally unattractive investment.
In 1938 the spinning band column was introduced for increasing fractionating efficiency. The first spinning band columns used directly coupled motors to rotate the band. In fractionating at below atmospheric pressure, leakage at the seals required for the direct coupling was a problem. As disclosed in U.S. Pat. No. 2,783,401 to Foster et al., magnetic coupling to an overhead motor avoided the seal problem. The arrangement remained cumbersome and expensive.
The bands themselves have been made mostly of metal. Various configurations have included metal strips, helical configurations and various disc shapes. More recently plastic bands have come into common use. U.S. Pat. No. 3,372,095 to Nester, discloses a band made of a rod of polytetrafluoroethylene wrapped with a spiral member of the same material. By making the spacing between the spiral member and the inner surface of the column small, the band is prevented from wobbling.
The overhead drive motor and an upper bearing for the spinning band necessitated by the drive arrangement has kept the expense high.
The present invention provides a novel low cost microdistillation column devoid of stopcocks and utilizing a plastic spinning band. The column achieves height equivalent/theoretical plate values approaching 0.5 cm/plate within 90 minutes of boilup.
The band utilizes a bottom magnetic drive system located in the distillation pot (still). Drive power is provided by conventional magnetic stirring plates. Thus, this system offers for the first time a very inexpensive distillation route to the separation of small quantities of low boiling liquid mixtures.